There are a number of various extracorporeal blood treatments, such as hemodialysis, hemofiltration, hemodiafiltration and plasmapheresis. They all make use of a filter and are often called dialysis. A conventional filter comprises a first and a second compartment separated by a membrane, the first compartment having an inlet and an outlet for the circulation of blood there through and the second compartment having an outlet for draining a liquid (e.g. plasma water, plasma, used dialysis liquid). In case the treatment (e.g. hemodialysis) requires the circulation of a treatment liquid (e.g. a dialysis liquid) the second compartment also has an inlet.
In the above treatments, blood is withdrawn from the patient, flown through the first compartment of the filter, and returned to the patient. This part is the so called extracorporeal circuit. In hemodialysis, a dialysis liquid is simultaneously flown through the second compartment of the filter and the metabolic wastes contained in the blood migrate by diffusion through the membrane into the second compartment. In hemofiltration, a pressure difference is created across the membrane so that plasma water flows through the membrane into the second compartment. Metabolic wastes migrate by convection into the second compartment. In order to compensate for the loss of body fluid, the patient is simultaneously infused with a sterile substitution solution. Hemodiafiltration is a combination of hemodialysis and hemofiltration. In this treatment a dialysis liquid is flown through the second compartment and a substitution solution is infused into the patient. In plasmapheresis, a pressure difference is created across the membrane so that plasma (i.e. plasma water and proteins) flows through the membrane into the second compartment. Once treated, the plasma is returned to the patient.
A machine for performing any of the above treatments comprises a pump for withdrawing blood from the patient through a so called arterial line connected at one end to the vascular circuit of the patient and at the other end to the inlet of the first compartment of a filter. The blood is pumped through the filter and is returned to the patient via a so called venous line connected at one end to the outlet of the first compartment of the filter and at the other end to the vascular circuit of the patient. The treatment machine also usually comprises a first blood pressure sensor for measuring the pressure of blood in the arterial line upstream of the pump, a second blood pressure sensor for measuring the pressure of blood in the venous line, a bubble detector for detecting any air bubbles in the venous line and a clamp for closing the venous line, for example when an air bubble is detected by the bubble detector. Sometimes also a third blood pressure sensor is present for measuring the pressure of blood in the arterial line downstream of the pump.
As is known by the man skilled in the art it is important to thoroughly watch the patient and the system during the treatment, as e.g. by measuring the blood pressure at a number of different points in the system. It is also known that it may be fatal to the patient if air is introduced into his/her blood system. Air might have come into the system via unintentional leaks.
The lines used for each dialysis treatment are disposable, since they are flown through by body fluids, such as blood, and are regarded as contaminated. The pressure sensors used in this field are for the same reason often separated into two distinct parts, one blood side part and one transducer side part. The actual pressure transducer, which is often expensive, is then protected from direct contact with the blood and can thus be reused, while the part that has been in contact with the blood is disposable.
A pressure sensing device according to the above is known from WO 2005/044340. In this document it is also mentioned that a permanent blood-air interface causes blood clotting. In order to reduce the risk of blood clotting the extracorporeal circuit should be so designed as to reduce the blood-air interface.
A connecting structure for an extracorporeal blood treatment machine is also disclosed in WO 2005/044340.